The present invention relates generally to cement compositions useful for cementing in low temperature environments and, specifically, to a dry cement composition for use in preparing a pumpable slurry for cementing pipe in wells drilled through low temperature earth formations.
In Artic regions, the surface temperature is so cold that underlying formations often do not reach 32.degree. F. for several hundred feet. These regions are termed "permafrost" regions meaning any permanently frozen subsurface formation. The frozen section may exist from a few feet to depths of 1500 feet or more. Permafrost varies from about 10% to as much as 90% ice and temperatures typically range from 8.degree. F. to about 15.degree. F. at depths of 25 to 100 feet. Cementing well conductor pipe and surface pipe in permafrost regions presents special problems. A cement slurry that bonds pipe to ice must fulfill a number of requirements before a good primary cementing job can be successfully completed. A neat Portland cement will not set up and provide strength in permafrost regions before it freezes.
One technique which has been used to combat this problem in hardrock areas involves heating the hole with warm drilling mud, then cementing with heated cement and heated mixing water. The heating delays freezing of the cement until after it has set up. This method cannot be used, however, when drilling through unconsolidated formations held together by ice because the hole will enlarge and create a void between the cement and formation. An acceptable permafrost cement must provide a low heat of hydration to reduce the enlargement of the hole due to melting of the ice formation. The slurry must develop a set product instead of freezing of temperatures of about 15.degree. F. to 32.degree. F. The slurry must, therefore, provide adequate compressive strength for supporting casing and provide good bonding to the ice formation. The slurry must also provide an adequate pumping time with a reasonable viscosity to allow placement of a uniform sheath of cement thick enough to minimize further thawing.
Gypsum-Portland cement blends have been used in the past to cement permafrost regions which blends typically contain a dispersant, a retarder and a freezing point dispersant. The preferred freezing point depressants used in the past were monovalent chloride salts with the preferred salt being sodium chloride, although potassium chloride was used at times. The freezing point depressant operated to lower the freezing point of the water used to form the cement slurry so that it would not freeze during mixing and curing of the cement.
It has now been discovered that a cement composition can be provided for use in cementing permafrost formations which utilize a divalent chloride salt and which has superior cementing properties.
It has also been discovered that a high yield slurry can be provided which allows the use of a high water concentration and provides a low slurry density which will develop adequate compressive strength at temperatures in the range of about 15.degree.-20.degree. F. without freezing. The system allows use of unheated mixing water and can be used with calcium aluminate cements.